burgerMenu requires pressing tab-key twice for keyboard navigation

[harshvchawla]

Steps to reproduce:

1. Open burgermenu
2. Click tab key to navigate through the menu items

Expected result:
Each menu item is jumped one-by-one on each tabkey press

Actual result:
Each jump requires 2 tab key clicks

Observations/Speculation:
burgerMenu sets tabindex=0 to its list items; and each list item is composed of <li><a>item</a></li> tags - so both <li> & <a> tags have their tabbing enabled
Even if we set <li>'s tabindex=-1 to disable it, burgerMenu package overwrites it everytime the menu is slid open